cmd/compile/internal/gc: minor cleanup of init.go comments

cmd/compile/internal/gc: minor cleanup of init.go comments

Step 5 was deleted in f3575a9 however the numbering of the other
steps wasn't adjusted accordingly.

While we're here: clean up the whitespace, add curly braces where
appropriate and delete semicolons.

Change-Id: I4e77b2d3ee8460abe4bfb993674f83e35be8ff17
Reviewed-on: https://go-review.googlesource.com/23066
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>